In a general syringe, a liquid medicine is inhaled into its syringe barrel when it is used, but recently a prefilled syringe in which the liquid medicine is previously filled in the syringe barrel was developed to reduce a working load at a medical site. A prefilled syringe in which a contrast medium is filled in its syringe barrel has also come into use lately. Moreover, a prefilled plastic syringe in which a contrast medium is previously filled in its plastic syringe having comparatively large capacity is also used these days.
When such a syringe is used, a plunger is inserted from a rear end of the syringe barrel and a screw portion formed at a tip of the plunger is screwed in a rear face of a gasket so that the gasket is mounted at the tip of the plunger. The gasket is thus mounted at the tip of the plunger and the gasket is pushed via the plunger, which causes the liquid medicine to be injected into a body or the like. Further, since the contrast medium has comparatively high viscosity and resistance when the contrast medium is injected into the body via a blood-vessel, spinal cord, or the like is large, pressure injection is generally performed using a device such as an auto-injector in a case of using the syringe in which the contrast medium is filled.
When the gasket is mounted at the tip of the plunger in this way, if a center axis of the syringe barrel and a center axis of the plunger are not in a state of coinciding with each other, the screw portion at the tip of the plunger cannot be smoothly screwed into the rear face of the gasket. However, it is not easy to make the center axis of the syringe barrel and the center axis of the plunger coincide with each other and the screw portion at the tip of the plunger is sometimes forced to be screwed into the rear face of the gasket in a state in which both of the center axes deviate from each other.
However, if the gasket is mounted on the screw portion at the tip of the plunger in the state in which the center axis of the syringe barrel and the center axis of the plunger deviate from each other, the gasket at the tip of the plunger becomes in a skewed position (a position in which the center axis of the gasket deviates from the center axis of the plunger) and the gasket becomes unable to smoothly move in the syringe barrel, which causes a problem that performance in filling the liquid medicine deteriorates. In addition, as a result of the gasket being in the skewed position, clearance is made between an inner surface of the syringe barrel and an outer circumferential surface of the gasket and fluid leakage easily occurs from the clearance.
It is an object of the present invention to provide a plunger for a syringe in which a gasket can be mounted on a screw portion at a tip of the plunger in a state in which a center axis of a syringe barrel and a center axis of the plunger are easily made to coincide with each other.